This invention relates to transparent miscible blends of polymers suitable for use in optical articles. This invention further relates to optical articles, and methods for making optical articles from the transparent blends.
In addition to use as optical articles, the blends of the invention are useful in producing transparent articles of manufacture having good properties. These properties include food chemical resistance and melt processibility. These blends are especially useful for making transparent molded articles, fibers, films and sheeting.
Polycarbonates and other polymer materials are utilized in optical data storage media, such as compact disks. In optical data storage media, it is critical that polycarbonate resins have good performance characteristics such as transparency, low water affinity, good processibility, good heat resistance, and low birefringence. High birefringence is particularly undesirable in high density optical data storage media.
Improvements in optical data storage media, including increased data storage density, are highly desirable, and achievement of such improvements is expected to improve well established and new computer technology such as read only, write once, rewritable, digital versatile, and magneto-optical (MO) disks.
In the case of CD-ROM technology, the information to be read is imprinted directly into a moldable, transparent plastic material, such as bisphenol A (BPA) polycarbonate. The information is stored in the form of shallow pits embossed in a polymer surface. The surface is sputtered with a reflective metallic film, and the digital information, represented by the position and length of the pits, is read optically with a focused low power (5 mW) laser beam.
The operating principle in a write once read many (WORM) drive is to use a focused laser beam (20-40 mW) to make a permanent mark on a thin film on a disk. The information is then read out as a change in the optical properties of the disk, e.g., reflectivity or absorbance.
Although the CD-ROM and WORM formats have been successfully developed and are well suited for particular applications, the computer industry is focusing on erasable media for optical storage (EODs). There are two types of EODs: phase change (PC) and magneto-optic (MO). In MO storage, a bit of information is stored as a −1 μm diameter magnetic domain, which has its magnetization either up or down. The information can be read by monitoring the rotation of the plane polarization of light reflected from the surface of the magnetic film. This rotation, called the Magneto-Optic Kerr Effect (MOKE) is typically less than 0.5 degrees. The materials for MO storage are generally amorphous alloys of the rare earth and transition metals.
Amorphous materials have a distinct advantage in MO storage as they do not suffer from “grain noise”, spurious variations in the plane of polarization of reflected light caused by randomness in the orientation of grains in a polycrystalline film. Bits are written by heating above the Curie point, Tc, and cooling in the presence of a magnetic field, a process known as thermomagnetic writing. In the phase-change material, information is stored in regions that are different phases, typically amorphous and crystalline. These films are usually alloys or compounds of tellurium which can be quenched into the amorphous state by melting and rapidly cooling. The film is initially crystallized by heating it above the crystallization temperature. In most of these materials, the crystallization temperature is close to the glass transition temperature. When the film is heated with a short, high power focused laser pulse, the film can be melted and quenched to the amorphous state. The amorphized spot can represent a digital “1” or a bit of information. The information is read by scanning it with the same laser, set at a lower power, and monitoring the reflectivity.
In the case of WORM and EOD technology, the recording layer is separated from the environment by a transparent, non-interfering shielding layer. Materials selected for such “read through” optical data storage applications must have outstanding physical properties, such as moldability, ductility, a level of robustness compatible with popular use, resistance to deformation when exposed to high heat or high humidity, either alone or in combination. The materials should also interfere minimally with the passage of laser light through the medium when information is being retrieved from or added to the storage device.
As data storage densities are increased in optical data storage media to accommodate newer technologies, such as digital versatile disks (DVD), recordable and rewritable digital versatile disks (DVD-R and DVD-RW), high density digital versatile disks (HD-DVD), digital video recorders (DVR), and higher density data disks for short or long term data archives, the design requirements for the transparent plastic component of the optical data storage devices have become increasingly stringent. In many of these applications, previously employed polycarbonate materials, such as BPA polycarbonate materials, are inadequate. Materials displaying lower birefringence at current, and in the future progressively shorter “reading and writing” wavelengths have been the object of intense efforts in the field of optical data storage devices.
Low birefringence alone will not satisfy all of the design requirements for the use of a material in optical data storage media; high transparency, heat resistance, low water absorption, ductility, high purity and few inhomogeneities or particulates are also required. Currently employed materials are found to be lacking in one or more of these characteristics, and new materials are required in order to achieve higher data storage densities in optical data storage media. In addition, new materials possessing improved optical properties are anticipated to be of general utility in the production of other optical articles, such as lenses, gratings, beam splitters and the like.
In applications requiring higher storage density, the properties of low birefringence and low water absorption in the polymer material from which the optical article is fabricated become even more critical. In order to achieve higher data storage density, low birefringence is necessary so as to minimally interfere with the laser beam as it passes through the optical article, for example a compact disk.
Another critical property needed for high data storage density applications is disk flatness. The disk flatness is dependent upon the flatness of the polycarbonate substrate immediately after the injection molding process as well as the dimensional stability of the substrate upon exposure to high humidity environments. It is known that excessive moisture absorption results in disk skewing which in turn leads to reduced reliability. Since the bulk of the disk is comprised of the polymer material, the flatness of the disk depends on the low water solubility and low rate of water diffusion into the polymeric material. In addition, the polymer should be easily processed in order to product high quality disks through injection molding.
There exists a need for compositions having good optical properties and good processibility and which are suitable for use in high density optical recording media. Polycarbonates manufactured by copolymerizing the aforementioned aromatic dihydroxy compounds, such as BPA, with other monomers, such as 6,6′-dihydroxy-3,3,3′,3′-tetramethylspirobiindane (SBI), may produce acceptable birefringence; however the glass transition temperature (Tg) melt viscosity is often too high, resulting in poor processing characteristics. Consequently, the obtained moldings have low impact resistance and low pit replication. Further, the water absorption of such polycarbonates is unacceptable for higher density applications.